This invention relates to an overcurrent protective device for the protection of electrical equipment from electrical overcurrent; and more specifically, to an overcurrent protective device which is equipped with a function of preventing the overheating of the electrical machinery.
FIG. 12 shows the construction of an overcurrent protector or motor protector of the prior art used for the protection of the motor typically in a tightly sealed compressor. The motor protector has an open ended cylindrical casing 10 made of an insulating material with first and second generally U-shaped terminals 12 and 14 extending from the interior of the casing through a top upper surface 10a of the casing to the exterior of the casing.
Lower ends of terminals 12 and 14 within casing 10 are bent approximately at a right angle as compared with the central axis of the casing thereby forming fixed contact 16 and 18 for the protector. A bolt 19 is secured within the casing with one end extending from the center of the upper surface 10a of the casing, and the other end extending into the interior of the casing. A shaft 19a extends from the bottom or the other end of the bolt 19 through a washer 22 on which a disk-shaped bimetal element 20 is installed. On the upper surface of the bimetal element 20 near its periphery, movable contacts 24 and 26 are connected as by welding or the like at positions respectively for making contact with fixed contacts 16 and 18.
In a position of normal operation, the bimetal element 20 is located at a first position where the disk periphery is curved upward with a fulcrum 19a as the center, and the movable contacts 24 and 26 are biased into contact with the fixed contacts 16 and 18 respectively. In this closed state, the electric current that has entered from the terminal 12 flows to the terminal 14 through the fixed contact 16, movable contact 24, bimetal 20, movable contact 26 and the fixed contact 18.
A resistance heater 28 made of nichrome wire is positioned in the casing 10 at a surface opposite to the upper surface 10a of the casing 10, and generally adjacent the upper surface of the bimetal element 20. As is shown in FIG. 13, resistance heater 28 extends between terminal 14 and a third terminal 15 with a generally rounded shape so as to heat the entire surface of the bimetal 20 as uniformly as possible between the second terminal 14 and the third terminal 15. There is an electric current path that flows from the second terminal 14 through the resistance heater 28 and exits from the third terminal 15 to outside.
The open ended bottom of the casing 10 is closed by a plate 30 made of an insulating material or a metal. This cover plate 30 is for the purpose of securing an electric distance between the object on which this motor protector is installed, such as the surface of a main compressor 32 and the contact part of the switch mechanism, and for preventing any foreign substance from entering the casing 10.
FIG. 14 shows an electrical circuit diagram with the motor protector connected to a motor M. The first terminal 12 is connected to one of the terminals of an electric source 34. The motor 36 in the compressor is connected between the other terminal of the electric source 34 and the third terminal 15. In the absence of a resistance heater 28 inside the motor protector, the terminal 36a of the motor 36 is connected to the second terminal 14.
When the switch of the motor protector is in a closed state, the electric current that flows to the motor 36 also flows to the bimetal 20 and the heating resistor 28, and the bimetal 20 is heated by the resistance heat generated in itself and the resistance heater 28. In addition, the bimetal 20 is also heated by the radiant heat from the compressor 32. The extent of this radiant heating is relatively small, however, as compared with the heating due to the aforementioned resistance heating.
Ordinarily, the motor 36 of the compressor 32 requires protection in the case where the electric current has exceeded a rated value because of an overload or a locked rotor state. For example, in the case where the cooling efficiency of a condenser (not shown in the drawing) has decreased appreciably, the amount of work done by the compressor 32 or the load on the motor 36 becomes excessively large, with a result that an overcurrent flows thereby creating a situation where the motor coils may be damaged.
Additionally, in the case where operation of the compressor is started again after the compressor 32 has once stopped and coolant gas of high temperature and high pressure still remains on the exterior side of the compressor, the piston is unable to compress the coolant gas; and thus, there is an abnormal increase in the amount of the electric current to start the motor.
In these cases where the electric current that flows to the motor 36 is increased as described above, heating is increased by the resistance heating which is transmitted to the bimetal element 20 with a result that the temperature of the bimetal element 20 rises. When the temperature rises to a prescribed action temperature such as 160 degrees centigrade, the bimetal element snaps to the second position where the disk periphery curves downward as shown by a dotted line 20' in FIGS. 12 and 14. Thereupon, the movable contacts 24 and 26 which are fixed to the upper surface of the bimetal element 20 are separated from the fixed contacts 16 and 18 respectively; and the switch circuit of the motor protector opens and the electric current is shut off. Because of this shutoff of the electric current, any possible harm to the coil of the motor 36 is prevented.
When the electric current is shut off, heating by resistance heating for the bimetal element 20 stops. When the temperature of the bimetal element 20 is lowered to a prescribed temperature such as, for instance, 80 degrees centigrade, the bimetal 20 snaps from the second position back to the first position, thereby closing the switch circuit. As a result of this action, the electric current flows and the operation of the compressor 32 starts once again.
There are, however, other cases where the motor 36 of the compressor 32 requires protection. If, for example, the coolant leaks out of the compressor due to some reason such as defective installation, there will be a reduction in the amount of the coolant gas of low temperature and low pressure that is circulated and supplied to the compressor 32 from an evaporator in the compressor with a resultant reduction in the cooling effect of the coolant gas in the compressor 32 or motor 36 and; thus, the temperature of the compressor 32 or the motor 36 rises. Since the amount of the coolant gas is small in such a case, the amount of work done by the compressor 32 is reduced, and the electric current that flows is not large enough to be above the rated value of the bimetal element. That is, even though the bimetal element 20 receives a radiant heat from the compressor 32 because of the above, the bimetal element does not snap over center because the main heating by the resistance heating is not sufficient to cause it to raise to a high enough temperature. Accordingly, the temperature of the compressor 32 or the motor 36 rises with a resultant possibility that the motor coils can be damaged.
To avoid the problem on certain prior art protectors, the bimetal 20 is constructed to snap in response to the radiant heat from the compressor 32, too. If the protector is designed in this manner, however, it becomes difficult to obtain the desired bimetal element action characteristics against the overcurrent at the time of an excessive load or locked rotor condition. That is, the simultaneous provision of an overcurrent protective function and an overheat preventive function in one bimetal switch device has been difficult to accomplish.
In an attempt to overcome this difficulty, a thermostat is often installed on the compressor 32, separately from the motor protector, for shutting off the electric current against an overheating of the compressor 32 as described above; and the switch circuit of this thermostat is connected in series with the switch circuit of the motor protector.
When such a thermostat is provided separately as described above; however, there is an increase in the manufacturing cost and two separate protective devices will be required for the overcurrent protection and overheating prevention, with a result that both handling and installation become increased.
All of these prior art approaches have deficiencies in one or more of performance, complexity, size or cost. Reliability and precise interaction of the various parts and switches has also proven to be of concern.